Investigation of Molecular Mechanisms Involved in Developmentally Regulated Programmed Cell Death in the Lace Plant (Aponogeton madagascariensis)
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Programmed cell death (PCD) is essential for the development and survival of eukaryotes. PCD is well understood in animals and relatively less is known in plants. PCD occurs throughout a plant’s life cycle, from the fertilization of ovule to the death of the whole plant. One of the fascinating examples of PCD in plant development is perforations formation in the leaves of lace plant (Aponogeton madagascariensis). PCD begins in the center of an areole of young leaves and develops towards the veins and stops approximately 4-5 cell layers from the vein, and these cells (NPCD cells) do not undergo PCD during perforation formation. Lace plant is an excellent system to work on developmentally regulated PCD because of the accessibility and predictability of perforation formation. Extensive morphological studies have been performed in the lace plant; however, less is known about the molecular mechanisms that drive lace plant PCD. The emphasis of this dissertation was to provide insights into the molecular mechanisms of developmentally regulated PCD in lace plant. We investigated the role of ethylene and ethylene receptors in lace plant PCD. Results suggested that ethylene is involved in lace plant PCD, in a climacteric-like pattern. Three lace plant ethylene receptors were isolated and their transcript expression pattern was studied throughout leaf development and between PCD and non-PCD (NPCD) cells. Based on the results, a newly proposed model in which ethylene and ethylene receptors regulate PCD and perforation formation was illustrated. The role of vacuolar processing enzymes (VPEs), in lace plant PCD was also investigated. Two lace plant VPEs were isolated and their transcript levels during leaf development and PCD were investigated. Results suggested that both VPEs are involved in lace plant PCD, but at different stages of leaf development. VPE activity analysis also suggested that VPE activity is higher in PCD compared to NPCD cells. Further, we investigated the effect of ethylene on the transcript expression pattern of VPEs. It was determined that ethylene plays a role in stimulating VPE transcriptional upregulation during lace plant PCD.